Voltage-sensitive dye imaging of action potentials (APs) in apical dendrites of layer 5 cortical pyramidal neurons. AP voltage waveforms were recorded simultaneously from 4 regions of interest along the apical axis. Backpropagation: AP evoked by current injection in the soma (patch) propagates from the soma into the apical tuft branches located in layer 1. Forward: AP evoked by glutamate iontophoresis in layer 1 (glut.) propagates from the apical tuft branch down to the cell body. (Image courtesy Srdjan Antic)